1. Field of the Invention
The present invention relates to an electrical steel sheet for a low-noise transformer, which lowers the vibration when the sheet is used for the core of a transformer or the like, and to a low-noise transformer.
2. Description of the Related Art
With respect to a magnetic material widely used in electrical and electronic apparatuses, the degree of a change in the length of the material when a magnetic field is imposed thereon (such degree of a change is called magnetostriction) is one of the important evaluation items in quality control since it causes transformer noise. In recent years, regulations against the noise of electrical apparatuses have been tightened with the increase in demands for better living environments. Because of this, research into the lowering of noise by reducing magnetostriction are being carried out intensively.
Among magnetic materials, as grain-oriented electrical steel sheets used for the cores of transformers, there is a method of reducing magnetostriction by decreasing closure domains. The closure domain cited here is a domain having magnetization oriented in a direction perpendicular to the direction where a magnetic field is imposed. Magnetostriction is generated when the magnetization moves toward a direction parallel to that of the magnetic field due to the imposed magnetic field. Therefore, the smaller the amount of closure domains is, the smaller the magnetostriction is. The following methods are known as major methods for reducing magnetostriction:
{circle around (1)} A method of arranging the <001> directions of crystal grains in the direction of rolling and preventing the generation of closure domains which cause a change in their shape due to magnetization rotation (T. Nozawa et al, “Relationship Between Total Losses under Tensile Stress in 3 Percent Si—Fe Single Crystals and Their Orientations near (110) [001],” IEEE Trans. on Mag., Vol. MAG-14, No.4, 1978),
{circle around (2)} A method of eliminating closure domains by releasing plastic strain (Japanese Unexamined Patent Publication No. H7-305115; “Development of Epoch-making Grain-Oriented Silicon Steel Sheet, Orient Core Hi-B”; OHM 1972.2),
{circle around (3)} A method of eliminating closure domains by imposing a film tension on a steel sheet (T. Nozawa et al, “Relationship between Total Losses under Tensile Stress in 3 Percent Si—Fe Single Crystals and Their Orientations near (110) [001],” IEEE Trans. on Mag., Vol. MAG-14, No.4, 1978).
On the other hand, noise can be lowered by the methods of suppressing the generation of vibration, besides the methods of reducing magnetostriction. The methods for lowering noise by suppressing the generation of vibration include, for example; a method of disposing an air space or a silicone rubber to cut off the propagation of vibration (Japanese Unexamined Patent Publication No. H5-251246), methods of lowering noise by disposing a vibration damping material and a sound absorbing material outside each core leg (Japanese Unexamined Patent Publication Nos. H8-45751, 2000-82622, and 2000-124044), a method of fixing the gap parts of a reactor by the use of an adhesive capable of suppressing vibration (Japanese Unexamined Patent Publication No. H8-111322), and a method of using an electrical steel sheet provided with an intermediate resin layer (Japanese Unexamined Patent Publication No. H8-250339).
The noise of electrical apparatuses have so far been lowered mainly by those methods of reducing magnetostriction or vibration.